Pulse Code Modulation (PCM) Viva questions
1. Mention channl noise?
The combined effect of
thermal noise, interference from other users, and interference from circuit
switching transients are termed channel noise. It can enter the transmission
path at any moment. It can be represented as Additive White Gaussian Noise
(AWGN) with a zero mean and power spectral density of No/2.
2. Discuss Inter Symbol Interference (ISI)?
A pulse waveform passing via a band-limited channel is
dispersed or spread. Spreading will exceed symbol duration when the channel
bandwidth is near to the signal bandwidth, causing signal pulses to overlap.
Inter Symbol Interference (ISI) is the term used for this
overlapping.
3. What do you know about PCM?
PCM is a type of baseband signal generated by encoding
each quantized sample into a digital word from quantized PAM signals. Sampling,
quantizing, and encoding are the three most important operations in the PCM
transmitter.
4. What is companding?
The process of signal compression and expansion is
known as companding. The compressor enhances weak signals and attenuates strong
signals at the transmitter. The expander performs the opposite of compression
at the receiver. The companding process is used to solve the problem of
quantization noise.
5. Discuss Line Coding?
Binary digits are coded into electrical pulses or
waveforms to deliver multiplexed message signals in the form of a sequence of
bits across a baseband channel. This
process is termed Line coding or transmission coding.
6. What do you know about the PCM
waveform? Discuss its types?
A
Pulse Code Modulation (PCM) waveform is generated when pulse modulation is
applied to binary symbols. These PCM waveforms are known as Line Codes in
telephone applications. The four types of PCM waveforms are as follows.
1. Non-Return to Zero (NRZ)
2. Return to Zero (RZ)
3. Phase encoded
4. Multilevel Binary
7. Illustrate about Non-Return to Zero (NRZ) waveform?
Non-Return to Zero (NRZ) waveforms occur when the PCM
waveform remains at any non-zero level for the whole bit interval T. The
following subgroups are:
(i) NRZ – L (L for level)
(ii) NRZ – M (M for mark)
(iii) NRZ – S (S for space)
8. Discuss about Return to Zero (RZ) waveform?
Return to Zero (RZ) waveform refers to a PCM waveform
that returns to zero after a portion of the bit interval T. It can be
classified into the subcategories listed below:
(i) Unipolar RZ
(ii) Bipolar RZ
(iii) RZ-AMI (AMI for Alternate Mark Inversion)
9. What is phase encoded waveform?
The time position of the occurrence or transition of a
pulse waveform is used to discriminate between various logic levels in a phase
encoded system. It can be classified as:
(i) Bi-φ-L (Bi-phase-Level)
or Manchester coding
(ii) Bi-φ-M (Bi-phase-Mark)
(iii) Bi-φ-S (Bi-phase-Space)
(iv) Delay Modulation (DM) or Miller Coding
10. What you know about Multilevel binary waveform?
Multilevel binary waveforms are binary waveforms that
encode binary data using three levels instead of two. The formats used in
multilevel binary are namely:
(i) Dicode NRZ
(ii) Dicode RZ
(iii) Duobinary signalling
(iv) Bipolar RZ
(v) RZ – AMI
11. Mention the factors that were utilised to choose a
particular PCM waveform type.
The most common criteria for assessing PCM waveforms
and determining a waveform type are as follows:
1. Spectral attributes or characteristics
2. Bit synchronization capabilities
3. Error detection performance
4. Interference and noise immunity
5. Cost and complexity or implementation
12. Interpret PCM wordsize.
The PCM wordsize is the number of bits necessary to
indicate the quantization levels of an analogue sample. Each speech sample has
256 quantization levels for digital telephone channels. As a result, PCM word
size is 8 bits (28 = 256).
13. List the different types of Pulse Modulation.
|
Analog
Pulse Modulation |
Digital
Pulse Modulation |
|
1.
Pulse Amplitude Modulation (PAM) |
1.
Pulse Code Modulation (PCM) |
|
2.
Pulse Width Modulation (PWM) |
2.
Differential Pulse Code Modulation (DPCM) |
|
3.
Pulse Position Modulation (PPM) |
3.
Delta Modulation (DM) |
|
|
4.
Adaptive Delta Modulation (ADM) |
14. Discuss in detail about M-ary Pulse Modulation
Waveforms.
The generated waveform is called M-ary Pulse
Modulation Waveform in pulse modulation schemes, when the output of the
pulse generator consists of pulses with M possible amplitude levels. M is
usually an integer power of two.
15. Mention the advantages and disadvantages of M-ary Pulse
Modulation schemes?
Advantages are:
1. The M-ary system, also known as multilayer
signalling, decreases the amount of bandwidth needed for transmission.
2. A high signal-to-noise ratio is important.
3. The M-ary technique is log2M times quicker than the
binary representation.
Disadvantages
1. In comparison to the binary scheme, the M-ary
strategy uses more energy to achieve equal detection performance.